Caravaggio was the greatest artist since Titian, a favourite of Popes and wealthy bankers.
But at a time when the resurgent Ottoman Empire was planning a second wave of conquest, he discovered a secret so dark that it threatened the very existence of the Catholic Church.
The secret endures. Four hundred years later, Declan O'Malley, the first Irish-born Superior General of the Society of Jesus, learns that his friend, the German Cardinal Horst Ruttgers, has died in mysterious circumstances.
With his nephew Liam Dempsey, recovering from wounds received while serving as a soldier with the United Nations, he tries to uncover the truth, bringing him into conflict with the sinister and virulently anti-Muslim Cardinal Bosani - Camerlengo, or High Chamberlain, of the Holy Roman Church - in charge of the upcoming Conclave to elect a new Pope.
As the two prelates grapple, Dempsey finds a bizarre link between Bosani and Caravaggio's masterpiece, "The Betrayal of Christ", lost for 200 years until it emerged in 1999 in the unlikely setting of the Jesuit house in Dublin. The painting turns out to be more than a sublime depiction of Christ's seizure in the Garden of Gethsemane; it is also the key to a centuries-old conspiracy of evil.
Can O'Malley and Dempsey, aided by the cool and resourceful Maya Studer, daughter of the Commandant of the Swiss Guard, prevent Bosani from re-igniting a calamitous war between Europe and the Muslim World?